This invention relates to the processing of pavements at roadways or the like and more particularly to a method and apparatus for more efficiently and economically repaving, repairing or heat treating pavements. In one form, the invention enables the recycling of existing asphalt roads at the roadbed using little or no new paving materials.
The introduction of the automobile as a major means of mass transportation and of automotive trucks as an important means of freight transport has been accompanied by a vast expansion of paved highway systems, roads, city streets and the like. Much of this road system is paved in whole or in part with asphaltic compositions and much of it is of fairly recent origin. Because of this historical situation, advances in asphalt paving technology have tended to be concentrated on techniques and equipment for new construction rather than on the repaving or repair of existing roads.
Asphalt pavement deterioates with age and as a result of heavy use. A variety of causes ranging from the concentrated localized stresses imposed on such pavement by heavy vehicles to ground settlement and ground water effects causes the pavement surface to become uneven, to crack and to exhibit so-called potholes in small localized areas where the pavement has for practical purposes decomposed. Repair or repaving efforts as heretofore practiced have tended to be essentially adaptations of new construction techniques. Initial repair steps typically consist of filling cracks, surface depressions and potholes with fresh new asphalt mix at the specific small areas in the pavement where this is needed. Patched areas of this kind often do not exhibit the stability and durability of the original pavement and thus repeated and usually more widespread patching becomes necessary in time. Eventually, when deterioration has progressed to a certain stage at least portions of the entire roadway may be repaved.
A typical road repaving operation of this kind has involved ripping up the old asphalt pavement which must then be loaded into trucks and transported to a dump site which is often a considerable distance away. New asphalt hot-mix is then brought in, laid in place and compacted in what is essentially a new paving operation with new material.
The fact that much of the vast existing asphalt roadway system is just now approaching a stage at which substantial repair or repaving will be needed, coupled with certain recent economic developments, is creating a potential crisis situation which has not been widely recognized until very recently.
A growing worldwide scarcity of petroleum derivative products, which includes asphalt, has very recently developed and has given rise to a substantial escalation of cost and perhaps a need to allocate the available resources of this kind between various competing needs such as energyproduction as opposed to road repair. When the high cost of new asphalt is combined with the labor, equipment and energy costs involved in repaving using known techniques, the resulting total cost figure appears to rule out any practical possibility of maintaining the vast highway system in the United States of America in a state approaching its present condition. Knowledgeable opinions have been expressed that deterioration of existing highway systems is, as a practical matter, almost a certainty. This situation will of course be further aggravated if the number of automobiles and trucks in use should be further increased as appears entirely likely.
To alleviate this situation by bringing the cost of highway repair down to a figure within the bounds of economic practicality, it has heretofore been proposed to recycle existing asphalt pavement. As heretofore contemplated and as has been carried out to a very limited extent, this involves ripping up the existing asphalt pavement on a road and transporting the chunks of old pavement to a distant processing plant where the pavement is heated to soften the asphalt. The constituents of the old pavement are then remixed. The remixed asphalt composition is then transported back to the roadway in a heated condition and utilized to repave the roadway in essentially the conventional manner.
This recycling of existing pavement realizes a substantial economy in that little or no new asphalt and aggregates is needed. An additional economic and ecological benefit arises from the fact that no dump site for old pavement may be required. Because of increasing ecological concerns and more intensive land use, suitable dump sites of this kind are becoming very difficult to find and if successfully located may be at great distances from the roadpaving site thereby further aggravating costs.
The economic savings of recycling asphalt paving in the known manner, as briefly discussed above, are largely confined to the savings realized by reducing or eliminating the need for new asphalt and aggregates. Under most circumstances at least, no substantial saving is made in the cost of transportation of materials nor in labor or equipment costs. Equipment costs at the processing plant may in fact increase somewhat as the type of apparatus used for heating new asphalt cannot usually be employed to heat recovered asphalt chunks, portions of which tend to ignite and to generate large quantities of smoke pollution if processed in the same apparatus used to prepare fresh hotmix. In one known recycling plant a specialized heat exchanger resembling a drum mixer is utilized in an arrangement which enables isolation of an external combustion chamber from the mix.
Partly to reduce ignition and pollution problems of the kind discussed above, it has very recently been proposed to use microwave energy for heating old asphalt pavement chunks at a recycling plant. Microwave energy is a flameless heating medium and is much more rapid than conventional techniques as it does not depend on conduction of heat inward from the surface of a pavement chunk but instead generates heat internally throughout the volume of the substance being heated. Use of microwave heating for such purposes is itself subject to many specialized technical problems which must be solved before such a procedure becomes practical. As one example, microwave tends to be a costly heating technique and therefore provisions must be made to assure that a high proportion of the generated microwave energy is converted into heat within the asphalt mix. As another example, it is essential that no sizable amount of microwave energy be radiated from the heating zone to the surrounding environment both for safety reasons and to avoid interference with other forms of electronic equipment such as radar systems, microwave communication links and the like. Techniques and equipment for utilizing microwave energy to heat other kinds of substance on a high-volume, continuous-process basis have heretofore been worked out, most notably in connection with the large-scale industrial processing of food products, and at least to some extent these known systems and procedures may be adaptable to reheating of used asphalt pavement in a fixed recycling plant of the kind discussed above.
Reference has been made above to the fact that the conventional repaving processes generally involve ripping up old asphalt pavement and transporting it to a dump site. Heretofore, this old asphalt pavement has not only been unutilized but has itself added substantially to the cost of a repaving operation because of the need to rip up the old pavement and the need for trucks to transport it to an often distant dump site and further because the dump site may then be rendered useless for other purposes. It has just recently been recognized that recycling of old asphalt pavement need not be confined to road or highway pavements. If practical and economic processes and equipment are available, the vast quantities of old asphalt presently occupying old dump sites can be converted to an asset. Such dump sites are essentially asphalt mines awaiting practical recovery techniques.
In general it may be said that it has just recently come to be recognized that the reuse of old asphalt pavement may not only offer sizable economies but may in fact be a practical necessity if the quality of existing road systems is to be maintained. At the same time, techniques and specialized equipment for accomplishing such recycling in a desirably efficient and economic manner have not heretofore been developed to an appreciable extent.
The foregoing discussion has dealt primarily with the economic disadvantages of prior techniques and systems for repaving or repairing asphaltic pavement. There are also other problems which are more technological than economic. One is that the bonding between new asphalt put down in cracks, low areas, potholes and the like and the older adjacent pavement tends to be relatively weak due to the fact that heating of the older pavement during the patching operations tends to be limited to just the exposed surfaces of the old pavement. A similar weak bond is likely to exist where a repaved section of road adjoins old pavement.
Still another problem is that hot-mix prepared at a plant for transport to a paving site must be overheated as some cooling during transit may occur. Further, such cooling tends to be uneven throughout the volume of hot-mix and serious temperature differentials may remain even if supplemental heating is provided en route to the paving site or at the site. Consequently, adjacent areas of pavement are put down and worked at significantly different temperatures with adverse effects on pavement quality.
Still another problem with conventional repaving and repair techniques results from the need to maintain hot-mix at high temperatures for long periods during transport and laying operations and in some cases to heat adjacent surfaces to which the asphalt is to be bonded. The practical problems of accomplishing this heating tend to rule out paving operations and all but the most serious repairs during periods of cold or wet weather. In many regions such activities must virtually stop during the winter.
The foregoing discussion has also dealt primarily with roads or other areas formed of asphaltic pavement. There are related serious problems with surfaces paved wholly or in part with other material. Much of the extensive freeway or throughway systems in the United States of America and elsewhere are basically concrete but very commonly these have shoulder strips paved with asphalt and which are subject to the problems discussed above. Moreover, concrete pavement itself deteriorates with use and age and may require an overlay or application of any of various materials accompanied by a heat treatment, which operations are also subject to very serious economic and technical problems of the general kind discussed above.